ANATOMY OF THE CELL. 15 
the new material is added in such a way that the completed 
wall is nearly uniform in thickness. On a cross-section through 
such a cell the limiting surfaces of the wall appear bounded by 
straight lines. (Fig. 8.) In the second class the walls are 
thickened irregularly, the new material being added only to 
certain portions, leaving thin places through which the liquid 
contents may pass easily and rapidly from cell to cell. This 
explains the use of the terms “finer” and “coarser structure”’ 
of cell wall. By “finer 
structure’’ is meant 
stratification or stria- 
tion or both. “ Coarser 
structure” refers to the 
more evident sculptur- 
ing seen on the walls 
of the latter class. 
In reference to its 
direction, growth in 
thickness may be either 
centripetal — toward 
the centre of the cell Cross section through the wood of Abies pectinata. TT 
tracheids, J year’s ring, H fall growth, F spring 
2 Or centrifugal rated growth, ¢ bordered pore, M medullary ray, m middle 
lamella, x 300. —( Wiesner.) 
Fia. 8. 
from the centre. In 
the latter case it must occur on that part of the wall not in 
connection with the protoplasm. It is evident that centrifugal 
growth, in this sense, can occur only in cells originally separ- 
ated from each other, and in those first connected by common 
walls which have later become partially separated by the 
division or splitting of their walls. 
The best example of centrifugal thickening of separate 
cells is the pollen grain, the outer surface of whose walls is 
often covered with projections of various forms. Among cells 
still partly united, examples occur where parts of certain cells 
of a tissue grow out into neighboring intercellular spaces. 
